For many years decorative, high pressure laminates have been used as a surfacing material in residential and commercial structures wherein aesthetic effects are desired in combination with functional behavior, such as wear, heat and stain resistance. Typical applications of said laminates are surfacing for walls, partitions, table tops, counter tops, furniture, doors and the like. These decorative laminates have been produced by a plurality of prior art processes. In making such laminates, it is conventional to utilize a plurality of core sheets generally composed of kraft paper which have been impregnated with a thermosetting phenolic resin.
It is generally desired to make laminates that range from about 1/32 inch to about 1/8 inch in thickness but, if desired, they may be made in even greater thicknesses. Accordingly, the thicker laminates require the use of a total number of core sheets comprising about three, five, seven, nine or even more. For decorative laminates, there is then placed on the stack of core sheets, with its decorative surface out of contact with the core sheets, a decorative overlay sheet which generally comprises a sheet of alpha-cellulose paper bearing a printed design or a light color thereon impregnated with a noble thermosetting resin. Resins for the decorative sheets are aminotriazine resins and additionally the unsaturated polyester resins, the epoxy resins and the like.
It is generally desirable, when making decorative laminates, to make use of a protective overlay sheet which is similar to the decorative sheet but generally devoid of design. It is placed atop the decorative sheet and in the final laminate is transparent.
Assemblies of this type may be individually laminated by application of heat and pressure thereto in a batch type process. However, for obvious economic reasons it is common practice to consolidate a plurality of these individual laminating assemblies into one large assembly, or press pack, and then to laminate this pack by heat and pressure application.
In building such a pack, an individual assembly is placed with its decorative overlayment surface adjacent to a highly polished stainless steel press plate. The function of the press plate is twofold. First, it provides a smooth, defect-free surface to one side of the laminate. Second, it serves to separate pairs of back-to-back assemblies, thus permitting a plurality of these assemblies to be consolidated into laminates in one operation, usually in back-to-back relationship.
In its simplest embodiment, a back-to-back press pack consists of this arrangement of one pair of individual laminating assemblies with a separator sheet between their core members. In actual commercial practice, however, the entire procedure is usually repeated many times, until a pack having a desired height has been built.
The press pack is then pressed or molded. This is accomplished by placing the pack between the platens of a hydraulic press heated with steam, hot water or other suitable heating medium under pressure. The press usually has multiple openings so that several packs may be pressed at once. A modern laminating press may have as many as twentytwo openings for sheet sizes up to 5 ft. .times. 12 ft. Since each opening can generally accommodate at least a ten-sheet pack, many laminates may be produced in each press cycle. When the pack has been placed between the press platen, pressure is applied until a net pressure in excess of 1,000 psi exists over the projected area of the sheets being laminated. With the pack under suitable pressure, the press temperature is raised by means of the fluid heating media which is introduced into the channels within the individual press platens. On a typical cycle, the temperature will rise to about 140.degree.C. within 30 minutes, remain at this level for 10 minutes to 30 minutes to accomplish curing of the resins, and return to room temperature in another 30 minutes. Cooling is accomplished by passing a cooling fluid through the same channels which guide the heating media or by simply cooling the heating media via a heat exchanger.
The word "curing" is a term frequently misused in the plastics industry. In molding and similar operations outside of the field of high-pressure laminates, curing refers to the transition of a resin from a soluble or fusible condition to an unsoluble-infusible condition by heat, chemical action or air drying. Thus when a liquid epoxy resin, for example, is poured into a mold and allowed to harden, it is said to be cured. In the manufacture of high-pressure laminates, curing takes place under heat and pressure in the platen press, as described hereinabove. However, after the fusion of the resin is accomplished, and the resin has become thermoset, there is an additional type of curing required which is more analogous to the annealing of glass. The laminate cannot be immediately removed from the press and allowed to cool rapidly in air because embrittlement of the surface layers will occur usually accompanied by warpage or buckling. As with newly cast or molded glass, the laminates must be slowly cooled by a gradual reduction of the heat and pressure to ambient and atmospheric conditions by gradually lowering the temperature of the platens.
After the curing is complete, the packs are removed from the press, the packs are disassembled and the laminates are sent on for finishing, while the press plates are returned for the next press run.
The use of controlled apparatus in processes for the production of laminates has in the past been limited to manual manipulation of the blow-off valve, application and regulation of the heat and the subsequent cooling of the press. This manual control of the laminating press has caused an increase in the cost of the resultant laminate because of the high cost of labor and has further increased the incidence of decreased quality and the need for more uniformity in products, due to the factor of human error which such a system, of necessity, inputs into the process.